Schneider et al. (2009a) used the pCO2 distribution and data for total nitrogen in the eastern Gotland Sea to estimate N2 fixation on the PI3K phosphorylation basis of mass balances. They hypothesized a spring N2 fixation that amounted to 74 mmol m−2, whereas 99 mmol m−2 was measured for the well-known summer fixation (Table 2). Because of the introduction of Cyaadd, our simulation resulted in almost the same spring N2 fixation (72 mmol m−2). But the model’s summer (June/July) N2 fixation by cyanobacteria

( Table 2) exceeded the mass balance estimate by 45% and was beyond the uncertainty range (20%) given by Schneider et al. (2009a). We suspect that the discrepancy was a consequence of different vertical integrations of N2 fixation. The mass balance was confined to the mixed layer, which had a depth of about 14 m during the cyanobacterial bloom. According to our model, however, the penetration of light controls the vertical distribution find more of N2 fixation and may stimulate N2 fixation well below 14 m. As a result, the model yielded an N2 fixation of 216 mmol m−2 for the entire period from April to July, whereas Schneider et al. (2009a) provided an estimate of 173 mmol m−2. In contrast to the mass-balance approach, our simulations also captured N2 fixation after the onset of mixed-layer deepening, which started in August. The contribution of this late

N2 fixation was 43 mmol m−2 resulting in a total annual N2 fixation of 259 mmol m−2 yr−1. In the base simulation, spring N2 fixation was negligible owing to the absence of Cyaadd. But since the total phosphate excess was still available in June, N2 fixation by cyanobacteria was large in June/July and continued more efficiently in the subsequent months. As a result, the total annual N2 fixation was almost identical in the two simulations. For ecosystem models, pCO2 is an extremely useful validation variable since it directly reflects the production of organic matter. This is especially important when the nutrient concentrations cannot be used to validate organic matter production because the elemental ratios (C : N, C : P) show large deviations from the Redfield ratios. By incorporating

the marine CO2 system Myosin into the model, we have shown that the parameterization of N2 fixation in the standard ERGOM needs to be modified. We cannot rule out another source for the missing nitrogen. Several model sensitivity tests (extending the model to include dissolved organic matter, different parameterizations of detritus etc.) were done, but they yielded no significant results. By applying a one-dimensional model to the station in the central Gotland Sea we miss all lateral effects. However, such an approach gives us the opportunity to model the main features of the system (like the seasonal variability of the surface nutrients, CO2 concentrations, primary production, temperature and other important processes for the CO2 surface cycle) and to elucidate the effect of single processes.